Mechanical and electrical consumer products are assemblies of parts. The quality and cost of the finished products depends heavily on the quality and cost of the individual components. It only takes one part not fitting properly to severely disrupt assembly, leading to unplanned increases in the time required to complete a product, with attendant increases in cost.
The biggest defense against this disruption is to make sure that all parts are perfectly correct before they are brought to the assembly floor. To do this perfectly requires 100% inspection, that is, a rigorous examination of at least the vital dimensions, which influence smooth, easy assembly.